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Description  |
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This invention relates to a solar collector comprising an elongate
absorber, including at least one duct for transferring heat derived from
solar radiation incident on the absorber to a heat transport medium in the
said duct during operation, the said absorber extending in the
longitudinal direction of an enveloping, sealed and evacuated transparent
tube which is provided, at least over a length portion of circular
cross-section, with an internal reflector of arcuate cross-section, the
absorber being arranged with its largest transverse dimension in the
symmetry plane of the internal reflector.
A solar collector of the kind set forth is known from the magazine
"Funkschau", 1975, No. 16, pages 19-20, notably FIG. 3.
The absorber of the known solar collector consists of a U-shaped tube whose
plane which extends through the axes of the legs of the U is situated in
the symmetry plane of the internal reflector between the longitudinal axis
of the enveloping transparent tube and the said reflector. The internal
reflector is provided over a region of at least 180.degree. on the inner
surface of the transparent tube.
Due to this asymmetrical construction, however, the manufacture of the
relevant solar collector is rather difficult. Moreover, the provision of a
reflective layer over half the inner surface of the cover tube is a time
consuming and expensive operation. Furthermore, in this solar collector
only part of the solar radiation is directly incident on the absorber, the
major part of the radiation being reflected to the absorber by the
internal reflector, so that reflection losses occur.
Moreover, if the tube is covered with a selective heat reflective layer
above the internal reflector, as in the known solar collector, part of the
heat radiation is reflected to the absorber only after multiple
reflections on the internal reflector and on the selective heat reflective
layer. Because any reflection involves losses, a comparatively high heat
radiation loss is incurred.
The invention has for its object to provide a high-efficiency solar
collector, which has a symmetrical and hence less costly construction and
which includes an inexpensive internal reflector.
In order to realize this object, the solar collector in accordance with the
invention is characterized in that the absorber is centrally arranged in
the tube and in that the reflector engages the absorber, the reflector
furthermore extending, in a cross-sectional view, through an angle of
120.degree. or substantially 120.degree. and intersecting the tube at
points which are situated at the ends of the diameter (X--X) of the tube
circle which extends perpendicularly relative to the reflector symmetry
plane (Y--Y), the centre (M) of the circle of which the reflector arc
forms part being situated on the reflector symmetry plane (Y--Y) such that
the circle radii (R) extend through the reflector/tube points of
intersection and are tangent to the absorber.
As a result of the central arrangement of the absorber in the enveloping
tube with the largest absorber transverse dimension in the symmetry plane
of the internal reflector, the portion of the solar radiation directly
incident on the absorber is increased, particularly in the case of oblique
incidence of radiation. Multiple reflections of the heat radiation prior
to being incident on the absorber occur to a smaller extent, so that the
heat radiation losses are comparatively low.
The shape of the internal reflector, a one-third circle cylinder, is
substantially optimally adapted to the central arrangement of the
absorber.
The internal reflector may consist of a flexible foil in accordance with
the invention. The internal reflector can be effectively connected to the
absorber. However, in a preferred embodiment, the internal reflector is
resiliently arranged in the transparent tube.
In a further preferred embodiment of the solar collector in accordance with
the invention, a flat reflector element is provided in the tube at each
end of the tube length portion of circular cross-section, the said
reflector elements extending transversely of the tube axis and also
serving to keep the absorber in position relative to the tube.
Each such element may consist of a thin aluminium sheet or of mica with a
vapour-deposited layer of aluminium or silver. These flat internal
reflectors serve to reflect the heat radiation originating from the
transparent tube and to reflect the solar radiation to the absorber in the
case of oblique incidence of radiation.
A still further preferred embodiment of the solar collector in accordance
with the invention is characterized in that the tube has dome-shaped end
faces, the inner surface of at least one of which is provided with a
reflective layer.
The reflective layer may consist of, for example, Al, Cu, Pb, Ag or Au and
is obtained, after evacuation and sealing of the tube, by induction
heating of quantity of metal provided on the respective flat reflector.
As a result of the heating, the metal evaporates and is deposited on the
inner surface of the respective end face of the tube in the form of a
reflective layer.
Another preferred embodiment of the solar collector in accordance with the
invention is characterized in that the inner surface of the tube is
provided with a light-transmitting but infrared reflective layer over at
least the length portion of circular cross-section which is situated on
the same side of the internal reflector as the absorber.
This layer may consist of, for example, gold, silver, tin dioxide or
tin-doped indium oxide.
Still another preferred embodiment of the solar collector in accordance
with the invention is characterized in that the absorber is provided with
a surface layer of a material having an absorption factor .alpha. .gtoreq.
0.85 for solar radiation and an emission factor .epsilon. .ltoreq. 0.3 for
heat radiation.
A selective absorbing layer of this kind may consist of, for example,
nickel oxide, copper oxide or cobalt oxide on a metal substrate.
The invention will now be described in detail with reference to the
accompanying drawings, in which:
FIG. 1 is a longitudinal sectional view of an embodiment of a solar
collector in accordance with the invention.
FIG. 2 is a cross-sectional view taken along the line II--II of FIG. 1.
FIG. 3 is a cross-sectional view of a further embodiment of the solar
collector.
FIG. 4 is a cross-sectional view of a still further embodiment of the solar
collector.
The reference numeral 1 in FIG. 1 denotes a transparent tube, in this case
of glass, having dome-shaped end faces 2 and 3.
On the end face 3 there is a sealed-off pumping stem 4 wherethrough the
interior of the tube 1 has been evacuated to a residual gas pressure of
less than 1 mbar. The inner surface of the tube 1 is covered over the
entire length of circular cross-section with a selective heat reflective
layer 5 of tin-doped In.sub.2 O.sub.3.
A tubular absorber 6 of glass or metal is sealed vacuum-tight centrally in
the end face 2 of the tube 1, the said absorber having a U-shaped
construction and comprising a duct 6a wherethrough a heat transport
medium, for example, water to be heated by solar radiation can be
conducted.
The axes Z of the two legs of the U-shaped absorber 6 are situated, as
appears from FIG. 2, in the symmetry plane Y--Y of an internal reflector 7
in the form of a thin flexible foil, for example, consisting of aluminium,
or of a synthetic material or mica provided with a reflective layer of
aluminium or silver, the length of the said foil corresponding
substantially to that of the absorber 6.
In a cross-sectional view, the foil 7 is shaped as an arc of a circle
extending over approximately 120.degree..
The foil 7 is loosely slid into the tube 1 and is retained therein in a
resilient manner. It contacts the absorber 6 substantially linearly.
The absorber 6 is provided with a black surface layer 8 of, for example,
glass enamel in the cylindrical portion of the tube 1.
The absorber 6 is secured in the tube 1 by way of two flat reflectors 9 and
10. These reflectors extend at the respective ends of the circular
cylindrical region of the tube 1, transversely of the tube axis, and are
made, for example, of mica with a vapour-deposited layer of aluminium. The
reflector 10 carries a getter ring 11. A glass projection 12 which engages
in the reflector 10 in a supporting manner is fused to the free end of the
absorber 6.
In order to prevent heat losses at the end face 2, the reflector 9 carries
quantities of metal 13, for example, aluminium, lead or copper.
After the tube 1 has been evacuated, it is subjected to external induction
heating in the region of the metal quantities 13, so that the metal 13
evaporates and is deposited against the inner surface of end face 2 of the
tube 1 in the form of a metallic reflective layer 13'. The evaporated
metal is also deposited against the absorber tube 6 in the region between
the reflector 9 and the end face 2. This is desirable to counteract
radiation of heat by the absorber tube in this region.
As is shown in the cross-sectional view of FIG. 2, the arcuate foil 7,
extending over a region of approximately 120.degree., intersects the tube
1 at points 14 which are situated at the ends of the diameter X--X of the
tube circle 1, the said diameter being perpendicular to the symmetry plane
Y--Y of the foil 7. The centre M of the circle of which the arcuate foil
forms part is situated on the symmetry plane Y--Y such that the circle
radii R extend through the points of intersection 14 and are tangent to
the absorber 6.
In a practical solar collector, the outer diameter of the tube 1 amounted
to 65 mm, the wall thickness to 1.2 mm, the overall length to 105 cm and
the length in the circular cylindrical region between the reflectors 9 and
10 to 98 cm. The selective heat reflective layer 5 of tin-doped indium
oxide had a layer thickness of 0.3 micron. The absorber 6 of glass had an
outer diameter of 17 mm and a wall thickness of 1 mm. The absorber surface
of black glass enamel had a thickness of 0.2 mm. The internal reflector 7
with a length of also approximately 98 cm and a width of 7.7 cm consisted
of an aluminium foil having a thickness of 0.1 mm.
The solar collector shown in FIG. 3 comprises an absorber 16 with flanges
16a which is completely made of metal and which is centrally sealed in the
glass tube 1 in a vacuum-tight manner. The duct 16b serves for a heat
transport medium which can discharge heat from the collector, if desired,
by an evaporation/condensation process. The condensate can be returned to
the absorber, if desired, by means of a capillary structure (not shown)
provided on the walls of the duct 16b (heat pipe principle; see, for
example, U.S. Pat. No. 3,229,759).
A thin internal reflector 17 is provided in the symmetry plane Y--Y, in
which the flanges 16a extend. The lower flange 16a contacts the reflector
17.
The points of intersection 14 of the reflector 17 with the circular tube 1
are again situated at the ends of the diameter X--X which is perpendicular
to the symmetry plane Y--Y of the arcuate reflector 17.
The centre M of the circle of which the arcuate reflector 17 forms part is
again situated on the symmetry plane Y--Y such that the circle radii R
extend through the points of intersection 14 and are tangent to the upper
flange 16a of the absorber 16.
The solar collector shown in FIG. 4 comprises an absorber which consists of
two concentric glass tubes 18 and 19 and which is centrally arranged
inside the glass tube 1, the absorber being sealed in a vacuum-tight
manner at one of the two ends (not shown) of the tube 1. The tube 18 is
sealed at the other end of the tube 1, the tube 19 being open at this
area. The duct 19a inside the tube 19 serves as a supply duct, and the
annular duct 18a serves as a discharge duct for heat transport medium.
The tube 18 is covered on its outer surface by a selective layer 20 which
readily absorbs solar radiation and which emits little heat radiation. The
layer 20 is made, for example, of cobalt sulphide on a metallic substrate
of, for example, silver.
The tube 1 again accommodates a thin arcuate reflector 21 which is
constructed as a one-third circle cylinder and which contacts the absorber
18, 19.
The description given with reference to location of the centre M on the
symmetry plane Y--Y of the circle including arcuate reflector 21.
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